1. Field of the Invention
The present invention relates to a development density adjusting method for an image forming apparatus such as a copying apparatus or a printer, and to an image forming apparatus.
2. Related Background Art
In the copying apparatus or printer of the electrophotographic process, the electrostatic image (electrostatic latent image) formed on a photosensitive member by imagewise exposure (image exposure) thereto has been developed by forming an electric field in the developing area and depositing developer onto the electrostatic image formed on the photosensitive member.
For forming such electric field, there is widely employed a rectangular wave bias voltage obtained by superposing a rectangular wave AC voltage with a DC component, because the rectangular wave can provide a large electric energy with a limited peak voltage.
The developer receives a force from the developer bearing member toward the photosensitive member by a flying voltage component in such bias voltage and also receives a returning force toward the developer bearing member by a returning voltage component, and these processes cause the developer to be deposited onto the electrostatic image on the photosensitive member, thus achieving the development.
Various commercial products utilizing the electrophotographic technology are provided with an image density adjusting device in order to enable the user to obtain a desired image, and such density adjustment is achieved by adjusting the amount of deposition of the developer in the developing process through the control of the bias voltage.
Among the conventional methods for controlling the bias voltage, there is already known a method of varying the magnitude of the DC voltage to be superposed with the rectangular wave AC voltage (conventional example 1).
FIG. 7 shows the level settings of the rectangular wave bias voltage, in the conventional example, for a maximum density F1, a standard density F5 and a minimum density F9, wherein Vmax indicates a development accelerating potential, Vmin indicates a returning potential, VL indicates a light potential corresponding to the image area on the photosensitive member, and Vd is a dark potential corresponding to the non-image area on the photosensitive member. Vpp is the peak-to-peak voltage of the bias voltage, and is always set at 1500 V.
In this method, a higher density image, for example, is obtained by increasing the flying voltage and decreasing the returning voltage, thereby enhancing the flying effect and increasing the deposited amount of the developer onto the photosensitive member.
In the illustrated example, a density increase for example from F5 to F1 is achieved by increasing the flying voltage .vertline.Vmax-VL.vertline. from 970 V to 1050 V and decreasing the returning voltage .vertline.Vmin-VL.vertline. from 530 V to 450 V. On the other hand, the development with a lower density is achieved by decreasing the flying voltage and increasing the returning voltage.
However, in such conventional example 1, the flying voltage and the reversal contrast tend to become large since the image density is adjusted by varying the magnitude of the flying voltage and the returning voltage.
For example, in the image development at a high image density, a high flying voltage causes the developer to be deposited only in the image area but also in the non-image area, thus causing so-called background fog (fog on background). Also in the image development at a low image density, the positively charged developer receives a large reversal contrast (difference between the returning potential and the dark potential of the photosensitive member) to result in a significant increase in the reversal fog (see. FIG. 6).
For example the reversal contrast becomes as high as 900 V at F1, 980 V at F5 and 1060 V at F9, thus resulting significant reversal fog at the low density side.
In contrast to such conventional example 1, there is also known a method of varying the image density by varying the ratio of the duration of the returning voltage to that of the flying voltage, while the magnitude of the flying voltage, returning voltage and DC component is fixed in the bias voltage.
In this method, the image density can be increased by extending the duration of the flying voltage with respect to that of the returning voltage, thereby increasing the amount of developer deposited onto the image bearing member.
FIG. 8 shows the settings, as conventional example 2, of the bias voltage for the maximum density F1, standard density F5 and minimum density F9. The potential settings (Vmax=-1300 V, Vmin=200 V, Vpp=-1500 V) are so selected as to allow comparison with the conventional example 1 and the embodiments of the present invention, under similar conditions.
In this method, the duty ratio, indicating the proportion of the duration of the flying voltage, is defined as follows: EQU Duty ratio=(Ta/(Ta+Tb)).times.100 (%) &lt;Formula 2&gt;
wherein
Ta: duration of flying voltage in a cycle of bias voltage PA1 Tb: duration of returning voltage in a cycle of bias voltage. PA1 forming a development area by opposing a developer bearing member bearing developer to an image bearing member bearing an electrostatic latent image; PA1 applying a voltage to the developer bearing member, wherein a value of the voltage periodically includes a first voltage value for forming an electric field adapted to direct the developer in a direction toward the image bearing member in the development area, and a second voltage value for forming an electric field adapted to direct the developer in a direction away from the image bearing member in the development area; and PA1 adjusting development density by varying ratio of application time of a voltage having the first voltage value to application time of a voltage having the second voltage value in one period, and difference between a potential of the developer bearing member and a potential of the electrostatic latent image, when the voltage having the first voltage value is applied to the developer bearing member. PA1 a) an image bearing member for bearing an electrostatic latent image; PA1 b) a developer bearing member opposed to the image bearing member to form a developing area; and PA1 c) voltage application means for applying a voltage to the developer bearing member, a value of the voltage periodically including a first voltage value for forming an electric field adapted to direct the developer in a direction toward the image bearing member in the development area, and a second voltage value for forming an electric field adapted to direct the developer in a direction away from the image bearing member in the development area;
The duty ratio is selected as 32.7% for F9; 38% for F5; and 43.3% for F1.
The conventional example 2 can suppress the increase in the background fog or the reversal fog, since the density is adjusted by a change in the duty ratio while the potential settings (Vmax=-1300 V; Vmin=200 V; Vpp=-1500 V) are fixed.
The conventional example 1 tends to result in a high flying voltage or a high reversal contrast, eventually leading to background fog or reversal fog.
On the other hand, the conventional example 2 is expected to provide an image with lower background fog or reversal fog than in the conventional example 1, since the flying voltage and the returning voltage are maintained constant so that the flying voltage or the reversal contrast does not become excessively high. However, as shown in FIG. 6, the conventional example 2 provides little fog at the low density side but shows a certain fog level at the high density side.
It will therefore be understood that the conventional example 2 cannot be the decisive means for sufficiently suppressing the background fog at the high density side, though it provides a higher flying voltage in the conventional example 1.
To catch the problem again, we will refer to the relation between the dimension of the difference between the flying voltage and the potential of the electrostatic image, and the ratio of the duration of the flying voltage to the duration of the returning voltage, referring to the wave form of the bias voltage.
In the wave form of the bias voltage, the area of the flying voltage can be defined, in the vertical direction, by the difference between the flying voltage and the potential of the electrostatic image and, in the horizontal direction, by the duration of the flying voltage. In the conventional example 1, the area at the level F1 is given by 1050 V in the vertical direction and 50% in the horizontal direction, while that in the conventional example 2 at the level F1 is given by 1150 V in the vertical direction and 43.3% in the horizontal direction. The amount of the developer flying to the photosensitive member is proportional to such area.
Referring to FIG. 6, the vertical magnitude of the wave form influences the fog more than the horizontal magnitude since the two conventional technologies provide a same image density but the conventional example 2 provides a higher fog level. Stated differently, for a same area of the flying voltage, namely for a same image density, a horizontally oblong wave form, with a reduced difference between the flying voltage and the potential of the electrostatic image and a longer duration of the flying voltage, is effective for suppressing the fog.
An increase in the image density is considered to be achieved, in the conventional example 1, by increasing the difference in the vertical direction between the flying voltage and the potential of the electrostatic image, but, in the conventional example 2, by extending the duration of the flying voltage in the horizontal direction. However a lower fog level can be obtained in the conventional example 2 than in the conventional example 1, because, as described above, the fog can be more effectively suppressed by reducing the difference between the flying voltage and the potential of the electrostatic image and extending the duration of the flying voltage.
However the increase of the developed density by extending the duration of the flying voltage in the horizontal direction alone is still insufficient, because, as shown in FIG. 6, the conventional example 2 still generates fog at the high density side.